One of the foremost considerations in the design of multi-channel station carrier systems is cost in order to make such equipment an economically attractive alternative to physical pairs. At the same time there is continuing need to improve the performance of station carrier systems.
One factor contributing to the cost of station carrier equipment is the use of tuned, narrow bandpass filters to sense a signalling tone which is transmitted over the called subscriber's channel to signal an incoming call for the subscriber. These tuned bandpass filters are customarily designed with one or more costly coils for recovering the transmitted signalling tone.
In the type of signalling operation employing such bandpass filters a relatively low frequency tone (e.g., 750Hz) is customarily modulated onto the carrier system's channel to be rung and is transmitted down the transmission line to the subscriber terminal equipment in the station carrier system. Each of the subscriber channel terminals in the carrier system is equipped with one of the narrow bandpass filters mentioned above to sense the low frequency tone when the tone is modulated onto its particular channel. Upon sensing the tone additional equipment is operated to ring the subscriber's telephone.
In such a system, annoying bell taps may be caused by transients emanating from lightning or other sources. To avoid bell taps some station carrier systems are equipped with circuits that suppress the transients, but not the low frequency signalling tone. Such circuits, however, add to the expense of the station carrier equipment.